1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke which acts to deactivate the catalyst. More specifically, this invention relates to cracking of FCC feedstreams in a transport contacting conduit.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to generally by those skilled in the art as "spent catalyst", is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
Despite the long existence of the FCC process, techniques are continually sought for improving product recovery both in terms of product quantity and composition, i.e. yield and selectivity. One facet of the FCC process that receives continued attention is the initial contacting of the FCC feed with the regenerated catalyst. Improvement in the initial feed and catalyst contacting tends to benefit yield and selectivity.
A variety of devices and piping arrangements have been employed to initially contact catalyst with feed. Most recent FCC arrangement contact catalyst in a riser conduit that transports the feed and catalyst upwardly in dilute phase as the reaction occurs. U.S. Pat. No. 5,017,343 is representative of devices that attempt to improve feed and catalyst contacting by maximizing feed dispersion. Another approach to improved feed and catalyst contacting is to increase the penetration of the feed into a flowing stream of catalyst. U.S. Pat. No. 4,960,503 exemplifies this approach where a plurality of nozzles surround an FCC riser to shoot feed into a moving catalyst stream from a multiplicity of discharge points. While these methods do improve distribution of the feed into the hot regenerated catalyst stream, there is still a transitory period of poor distribution when the relatively small quantities of the hydrocarbon feed disproportionately contact large quantities of hot catalyst. This poor thermal distribution results in non-selective cracking and the production of low value products such as dry gas.
The processing of increasingly heavier feeds and the tendency of such feeds to elevate coke production and yield undesirable products has led to new methods of contacting FCC feeds with catalyst. Of particular interest recently have been methods of contacting FCC catalyst for very short contact periods. U.S. Pat. No. 4,985,136 discloses an ultrashort contact time process for fluidized catalytic cracking, the contents of which are hereby incorporated by reference, that contacts an FCC feed with a falling curtain of catalyst for a contact time of less than 1 second and follows the contacting with a quick separation. U.S. Pat. No. 5,296,131, the contents of which are hereby incorporated by reference, discloses a similar ultrashort contact time process that uses an alternate falling catalyst curtain and separation arrangement. The ultrashort contact time system improves selectivity to gasoline while decreasing coke and dry gas production by using high activity catalyst that contacts the feed for a relatively short period of time. The inventions that provide short contact time are specifically directed to zeolite catalysts having high activity. The short contact time arrangements permit the use of much higher zeolite content catalysts that increase the usual 25-30% zeolite contents of the FCC catalyst to amounts as high as 40-60% zeolite in the cracking catalyst. These references teach that shorter hydrocarbon and catalyst contact time is compensated for by higher catalyst activity. Methods for ultrashort catalyst and feed contacting require unconventional contacting equipment and extensive replacement of existing equipment.
Many methods of ultrashort catalyst contacting perform an initial fast separation of the primary reacted products and collect the catalyst in a dense bed. The catalyst that enters the dense bed still contains a large amount of adsorbed and entrained hydrocarbons. The continued contacting of these hydrocarbons in a dense phase catalyst bed leads to overcracking of the remaining hydrocarbons and results in loss of products and the production of unwanted light gases.
The mixing of additional spent catalyst with the carbonized catalyst or the addition of catalyst to a traditional FCC riser arrangement or non-traditional short contact time arrangements have also been advantageously employed. U.S. Pat. No. 5,451,313 issued to Wegerer is an arrangement wherein regenerated and spent catalyst are mixed in a distinct chamber at the bottom of the riser and a secondary product stream is withdrawn from the riser. U.S. Pat. No. 5,858,207 issued to Lomas teaches the mixing of spent and regenerated catalyst at the bottom of the riser. The mixing of the regenerated and spent catalyst offers advantages of varying catalyst to oil ratios without the increase in catalyst temperature that occurs by the use of regenerated catalyst alone. In this regard spent catalyst has been found to have sufficient activity to be particularly useful in providing a blended catalyst mixture.
Therefore, improved or alternate methods are sought for ultrashort catalyst contacting. Improved methods will contact the feed using more conventional type equipment and with more traditional operations. Other improvements will focus on the better control of entrained and adsorbed hydrocarbons that are left on the catalyst.
It is an object of this invention to improve the control of cracking reaction time for light readily cracked hydrocarbons and more refractory heavy hydrocarbons that are adsorbed or otherwise entrained with catalyst.
Another object of this invention is to provide initial ultrashort contacting of feedstream in a transport conduit with continued controlled residence time cracking of adsorbed or entrained hydrocarbons that remain entrained with the catalyst after withdrawal of the initial product.
A further object of this arrangement is to provide a short contact time system that can be readily operated to provide more traditional contact times.